The present invention relates to a process for optimizing the value of natural gas.
The present invention is directed to a process for converting gas resources in remote locations to fuels and petrochemicals. Although the operating cost for producing fuels and petrochemicals from remote natural gas is significantly lower than producing the same products from oil, high capital costs discourage such conversion of natural gas. Reducing such capital costs would provide an incentive for utilizing natural gas as a relatively inexpensive source of fuels and petrochemicals.
Natural gas is often co-produced with oil in remote offsite locations where reinjection of the gas is either expensive or not feasible. A desirable option for treating such gas is its conversion to methanol, using Fischer-Tropsch technology, which is simpler than gas liquefaction. Methanol can be produced at reasonable cost in plants ranging from 500 to 50,000 tons a day. While larger plants have a large cost advantage, small plants are nevertheless viable if the gas cost is sufficiently negative, e.g., reinjection of natural gas is too costly or impossible. Accordingly, it has been suggested as economically feasible to place methanol plants on barges for offshore producing locations.
Despite such advantages, methanol""s market is limited to only about 100 million tons per year, significantly less than that which could be produced from all natural gas sources, and therefore not suited to the scale of co-produced natural gas. However, methanol can be converted to gasoline, olefins, or a mixture of both. Indeed, a commercial plant for converting natural gas to gasoline has operated in New Zealand. Such methanol conversion processes are generally designed as integrated large-scale plants, where natural gas is converted to methanol that is then converted to hydrocarbon products.
For offshore and other difficult gas producing locations, it is preferable to provide a process, which is simple to operate. Because methanol production from natural gas is the simplest way to convert gas to liquid, it is highly desirable for on-site use. However, methanol""s lack of a large-scale market militates against such conversion in the absence of a means to economically convert methanol to more readily marketable products.
U.S. Pat. No. 3,898,057 incorporated herein by reference, discloses a process for converting natural gas to a mixture of carbon monoxide and hydrogen at the site of production, converting the mixture to methanol, and transporting the methanol to a place of consumption where it is burnt or reconverted into methane.
Despite current practices, there exist certain advantages to totally separating the methanol conversion step from the production of methanol, and conducting such methanol conversion in a separate unit within a large existing refinery or in a large, dedicated methanol refinery, either of which is located in a region readily accessible to petrochemical markets, e.g., the Gulf Coast, Rotterdam, Singapore, etc.
A significant portion of costs associated with converting methanol is attributable to services or products supplied from outside sources such as steam, electricity, and fresh water. These are often available to a refinery more cheaply than at or near a natural gas producing site due to the refinery""s economies of scale as well as location with respect to the supply of such services or products. In addition, gasoline product tankage and distribution infrastructure is already available at the refinery as well as facilities to handle light gas by-products.
Another reason refineries are advantageous locales for converting methanol stems from their enhanced profitability by converting methanol to gasoline as well as other higher value products as compared to solely producing a product for its fuel value as is done where methane is converted to a liquid fuel at the production site for natural gas. For example, refineries can co-produce chemicals, such as ethylene, propylene, and aromatics, having higher value than motor fuels. Moreover, the location of many refineries near petrochemical complexes or petrochemical pipelines provides a readily accessible, proximal market for such chemicals. Transporting methanol from a remote location, say, at least 10 miles, at least 20 miles or even at least 100 miles, to a refinery located near chemical markets is less expensive than transporting petrochemicals from local conversion plants for ethylene or propylene near natural gas sources. Indeed, such ethylene or propylene made near the natural gas producing site can require grassroots polymerization facilities in order to render these products transportable. In contrast, a methanol refinery located near ethylene and propylene merchant markets can readily dispose of ethylene and propylene without further processing. In general, the methanol refinery of the present invention is advantageously located where a critical mass of other refineries and petrochemical plants already exist. The methanol refinery is dependent upon a supply of low cost methanol such as that which can be economically produced from gas fields having no access by pipeline to suitably sized markets.
Decoupling gas conversion to methanol from expensive methanol upgrading processes that require investments significantly greater than the cost of the gas conversion plant provides additional benefits. Integrating methanol conversion into a refinery is particularly advantageous where the refinery can vary its product distribution according to market demand or refinery requirements. Such flexibility is lacking where methanol is converted in a remote location such as a natural gas production site.
Although a single natural gas producing site may not economically justify a methanol conversion plant in addition to methane conversion, a single methanol conversion plant could service multiple remote natural gas producing sites that transport their site-produced methanol to the plant. Such an arrangement would allow the use of a larger-scale methanol conversion plant with its attendant economies. Thus a properly situated methanol refinery would create a market for smaller natural gas producing sites that are individually unable to support individual dedicated methanol conversion plants.
According to the invention it has now been found that natural gas containing methane can be converted to higher value products by a process which comprises: i) converting said methane to methanol at or near a site of gas production; ii) transporting said methanol to a refinery remote from said site of gas production (say by at least 10 miles, at least 20 miles, at least 100 miles, or even at least 1000 miles) and proximal to petrochemical markets (say within 100 miles, preferably within 20 miles, or even more preferably within 5 miles), said refinery producing ethylene and propylene product streams and comprising an alkylation unit, e.g., one which can utilize a propylene feed; and iii) converting said methanol to gasoline boiling range fuel product and petrochemicals, such as ethylene, propylene, butenes and xylenes.
In another aspect, the present invention further comprises substituting the butenes produced from methanol for at least some of the propylene feed (producible from crude oil), in the refinery""s alkylation unit to provide gasoline boiling range fuel product.
In yet another preferred embodiment, the present invention further comprises collecting individual streams of ethylene, propylene, and gasoline boiling range fuel product.
In yet another preferred embodiment, the present invention allows the refinery to produce larger quantities of gasoline low in sulfur and benzene. The C4+ stream produced from methanol conversion contains very low levels of sulfur, e.g., less than 10 ppm, preferably less than 5 ppm sulfur, and more preferably no sulfur at all. Such a stream can also contain very low levels of benzene, e.g., less than 2.5 weight percent, preferably less than 1 weight percent. Blending this clean stock with gasoline fractions derived from crude oil can allow existing refineries to operate more efficiently by reducing the amount of energy and processing required to produce gasoline. A C4+ gasoline boiling range product stream containing less than 10 ppm sulfur produced by the present process can be blended with a second gasoline boiling range product stream which contains at least 10 ppm sulfur to provide a gasoline boiling range product stream having a reduced sulfur content compared to said second gasoline boiling range product stream.
In another aspect, the present invention relates to a process for treating methane-containing natural gas which comprises: i) converting said methane to methanol at or near plural sites of natural gas production; ii) transporting said methanol from said plural sites to a single refinery remote from said sites of production, said refinery producing ethylene and propylene and comprising an alkylation unit; and iii) converting said methanol to gasoline boiling range fuel product and at least one petrochemical selected from the group consisting of ethylene, propylene, butenes and xylenes.
The above and other objects, features and advantages of the present invention will be better understood from the following detailed descriptions, taken in conjunction with the accompanying drawings, all of which are given by illustration only, and are not limitative of the present invention.